Revert "Merge branch 'bsall@bugfix-storage-error-during-transfer' into 'master'"

Context

!3913 (merged) should have been merged after !3867 (merged). The reason is that if Contract_storage.spend_only... is called with a null amount, then one could end up subsequently calling Storage.Contract.Balance.update on a non existent entry, which triggers an error. But with !3867 (merged), that is not possible.